This invention relates to wrapping apparatus for applying stretch wrappings to objects. The apparatus is usable in general industry, e.g. for wrapping palletised articles, but is primarily intended for use in agriculture.
In particular, the apparatus disclosed herein has been devised principally for use in the field for wrapping bales of agricultural produce such as silage, hay and straw, as an alternative-to bagging the bale. Often there is a need to wrap a bale in such a way that ingress of air or oxygen is eliminated substantially or completely. Wrapping film is costly and ultimately discarded, so there is a need to maximise its economic usage.
In agriculture a bale, e.g. of silage, is wrapped with stretch film by making numerous overlapping turns of film around the bale so as to form a completely-enveloping "bandage". This is accomplished by relative rotation between the bale and a film dispenser. The dispenser may be fixed. In that case, the bale is rotated about two axes until the necessary complete film coverage has been wound about the bale. Alternatively, the dispenser might be rotated about the bale while the latter is rotated appropriately, again until a complete film wrapping has been formed.
GB-A-2 159 489 discloses a bale-wrapping apparatus connectible to a farm tractor. The typical cylindrical bale is rotated horizontally about its main axis and about a vertical axis while a fixed dispenser pays out film from a film supply roll to the rotating bale. The film is drawn from the roll thanks to rotation of the bale to which an end of the film has been attached. Since the bale is rotating about two axes, in due course it becomes covered completely with overlapping turns of film. The film is applied to the bale under tension. That is, it is stretched while passing between the dispenser and the bale. In essence, this is achieved by the film being pulled by the rotating bale from the supply roll, the latter being braked in some way. The film is extended and stretched as it is drawn from the dispenser toward the bale. The amount of stretch depends on the surface velocity of the bale and the equivalent, slower speed of film leaving the film roll, the latter being dependent on the braking or retarding force developed at or by the dispenser to which the roll is mounted.
EP-A-0 242 975 discloses a similar apparatus. Film leaves the dispenser again due to the pull exerted by the rotating bale, which pull is resisted by a braking effort at the dispenser. In this case, the resistance is created by a controllably-resistive hydraulic circuit including a pump operated by the supply roll as it rotates when film is being drawn off its supply roll.
In both these prior apparatuses, film tension is developed between the bale and dispenser thanks to the pull exerted by the rotating bale being resisted by the dispenser.
This means of tensioning the film has severe practical drawbacks. In particular, the lengthwise film stretch is accompanied by a transverse shrinkage. This obviously is not beneficial if cost-effective film usage is desired. Moreover, the amount by which the film can be stretched by this tensioning method is limited. It is found that attempts to increase the stretch, by increasing the speed of rotation of the bale and/or by increasing the dispenser braking, are only partially successful. Most films can only be stretched this way by disappointingly small amounts and if they are exceeded, holes appear in the film and/or it may break. It is very difficult to match bale speed and dispenser braking for optimum film usage.
Nowadays, wrapping or encapsulating a bale to provide a degree of protection to the contents of the bale employs extensible and partially elastic or stretchable polymeric film. Films used are usually clear, white, black or some other color or combinations of colors. The films used may include an ultra violet light inhibitor to provide enhanced protection to the encapsulated material. Some films are provided with a tack or low level of self-adhesive on one surface or on both surfaces, to improve the sealing characteristics of the layers and joints on the bale.
Polymeric film is expensive and hence for this reason there is a desire to reduce the amount of film used to wrap a bale. Moreover, polymeric film is not readily biodegradable, and once the protective wrapping is removed, it is not easily reusable, so on environmental grounds there is again a need to maximise efficiency of film usage.
Film usage is likely to rise significantly through greater use in agriculture particularly for wrapping bales of silage. Currently, approximately only 30% of the UK silage crop is made in this way and the remaining 70% is still being made in environmentally hazardous outdoor clamps.
It is important to ensure that the encapsulation process is complete and thorough, when wrapping a bale of silage. Firstly the contents of the bale must be isolated from the atmosphere for the ensiling process to take place, turning the bale of raw grass into a quantity of high value long life animal feed. Secondly, seepage from the encapsulation of the contaminative, poisonous, acidic products that are produced as by-products at certain times during the ensiling process must be contained.
It will be understood, therefore, that when wrapping bales of silage it is of prime importance to ensure the best possible encapsulation of every single bale.
The present invention permits the application of film to a bale under tension, where control of tension is significantly better than has hitherto been economically possible. Prior film dispensing systems, where the film is pulled from a retarding film dispensing device often lead to film perforation and narrowing or `necking` of the film due to the occurrence of uncontrolled excess tension, and uneven tensioning. An unsatisfactory, uneven distribution of film over the surface of the bale results, which may well not be airtight or liquid-tight.
With systems commonly in use, inadequate film tensioning is also known to happen at times. Then, the film is applied too loosely, that is, without enough tension to cause it to adhere firstly to the bale and secondly to the preceding layers resulting in an unsealed package.
With currently available systems of the kind discussed above, only low percentage stretch or elongation can be obtained reliably so film usage is undesirably high. At best, stretch percentages may be only 40-70%.
It has been demonstrated that power stretching the film in the dispenser can elongate the film reliably, without significant transverse shrinking, holing or tearing problems, at up to 200% or more. Highly efficient film usage can thus be secured. In power stretching, the film from the supply roll is passed around several rollers and through the nip or nips of two or more of the rollers, selected ones of which are positively driven at appropriate surface speeds so as to stretch the film as it passes through the dispenser and before it is fed therefrom to the bale. However, the present inventor has found from long experience that it is necessary to inter-relate and thus control the drives to the power stretch dispenser and to the bale rotating means.
In his WO94/20367, the present inventor discloses a wrapping apparatus where the dispenser and bale rotating drives are hydraulic motors. The dispenser motor is driven off the exhaust of bale rotating motor and this makes it possible to control and fix the ratio of the outputs of the two motors. In this way, highly reliable and cost-efficient film usage is possible.
The system of WO94/20367 is of great simplicity but can be difficult in practice to optimise, especially when produced as a retrofit or accessory for farmers' existing tractors. There are numerous makes and types of tractor in existence, and large numbers of hydraulic supply systems, and all in different states of upkeep. Under the circumstances, "tuning" the system of WO94/20367 to any particular tractor, and maintaining its efficient time, can be extremely time consuming.
A related system developed from WO94/20367 is disclosed in PCT/GB96/01458. In this system, benefits are obtained by running the bale rotating motor off the exhaust of the dispenser motor, again obtaining a fixed ratio of the outputs of the two motors. More stable operation may be attainable with this system, the hydraulics of which have been developed to make the system more adjustable, Even so, this system is as difficult and time consuming to set up and maintain at peak efficiency as the former, and is more expensive.